1. Field of the Invention
This invention relates to an oxynitride film which comprises nitrogen and oxygen whose compositional ratio varies along the thickness of the film and a method for forming the film. The invention also relates to a method for forming an element isolation oxide film using the oxynitride film.
2. Description of the Related Art
In semiconductor integrated circuits, it is essential to form element isolation regions in order to avoid electric interference between the adjacent elements in operation and also to completely and independently control individual elements. For the formation of element isolation regions, the LOGOS (local oxidation of silicon) method has been hitherto known widely.
Reference is now made to FIGS. 115 to 117 for illustrating the LOGOS method which has been popularly, widely known in the art. FIGS. 115 to 117 are, respectively, sectional views showing first to third steps of the known LOCOS method.
As shown in FIG. 115, an about 500 .ANG. thick silicon oxide film 6 is formed on a main surface of a silicon substrate 1 according to a thermal oxidation method. Subsequently, an about 1000 .ANG. thick silicon nitride film 3 is further formed on the silicon oxide film 6 by a CVD (chemical vapor deposition) method or the like. In order to relax the stress of the silicon nitride film 3 exerted on the silicon substrate 1 at the time of selective oxidation, the silicon oxide film 6 should have a thickness which is at least half the thickness of the silicon nitride film 3.
Referring to FIG. 116, the silicon nitride film 3 is patterned in a given form according to photolithography and etching techniques. Using the patterned silicon nitride film 3 as a mask, selective oxidation is effected to selectively form an element isolation oxide film 4 on the main surface of the silicon substrate 1 as shown in FIG. 117. Thereafter, the silicon nitride film 3 is removed by use of hot phosphoric acid or the like, followed by removal of the silicon oxide film 6 by use of hydrofluoric acid or the like.
The element isolation oxide film 4 is formed through the above steps. However, the LOCOS method has the problems set out hereinbelow.
As shown in FIG. 117, the element isolation oxide film 4 has bird's beaks extending beneath the silicon nitride film 3. A greater length of the bird's beak (i.e., a length of the bird's beaks extending in parallel to the main surface of the silicon substrate 1) causes a greater deal of troubles concerning the high degree of integration of semiconductor integrated circuits. The bird's beak length is greatly influenced by the thickness of the silicon oxide film 6. More particularly, a greater thickness of the silicon oxide film 6 undesirably leads to a greater length of the bird's beaks. In the known LOCOS method, the silicon oxide film 6 should be formed in a thickness as large as approximately half the thickness of the silicon nitride 3, thus presenting the problem that the bird's peaks undesirably become prolonged.
In order to solve the above problem, many improvements have been proposed. One of such improvements is particularly shown in FIGS. 118 to 120. FIGS. 118 to 120 are, respectively, sectional views showing first to third steps of the LOCOS method proposed in Japanese Patent Laid-open No. 63-21848.
As shown in FIG. 118, a silicon oxide film 6 having a thickness of about 100 .ANG. to about 200 .ANG. is formed on the main surface of a silicon substrate 1 according to a thermal oxidation method. Then, an oxynitride film 8 having a thickness of about 200 .ANG. is formed on the silicon oxide film 6 according to a low pressure CVD method. The oxynitride film 8 is an oxygen atom-containing silicon nitride film and is intermediate between the silicon oxide film and the silicon nitride film with respect to the properties thereof. The oxynitride film 8 contains oxygen and nitrogen substantially uniformly along the thickness of the film. According to a low pressure CVD method, a silicon nitride film 3 having a thickness of approximately 1500 .ANG. is formed on the oxynitride film 8.
Referring to the FIG. 119, the silicon nitride film 3 and the oxynitride film 8 are patterned using photolithography and etching techniques. Subsequently, selective oxidation is carried out using the pattern of the silicon nitride film 3 and the oxynitride film 8 as a mask, thereby selectively forming an element isolation oxide film 4 on the main surface of the silicon substrate 1.
In the above improvement, the oxynitride film 8 is formed beneath the silicon nitride film 3. Since the oxynitride film 8 has intermediate properties between the silicon oxide film and the silicon nitride film as set out hereinabove, the oxynitride film 8 is softer than the silicon nitride film 3. This serves to relax the stress exerted on the silicon substrate 1 by the action of the silicon nitride film 3. As a result, it becomes possible that the thickness of the silicon oxide film 6 formed beneath the oxynitride film 8 is smaller than in the case of the afore-stated known LOCOS method. This eventually leads to a smaller length of the bird's beaks.
However, the above improvement has the following problems. The oxynitride film 8 used in the improvement contains oxygen and nitrogen substantially uniformly along the thickness thereof. This means that the oxynitride film 8 contains nitrogen in a substantially uniform amount not only in portions near the main surface of the silicon substrate 1 but also in portions distant from the silicon substrate 1. Accordingly, the stress of the silicon nitride film 3 exerted on the silicon substrate 1 is not relaxed satisfactorily.
In order to relax the stress of the silicon nitride film 3 and effectively prevent the formation of crystal defects in the main surface of the silicon substrate 1, it is necessary to form a silicon oxide film 6. The silicon oxide film should have a relatively large thickness although the thickness is not so large as required in the known LOCOS method. Thus, the improvement is disadvantageous in that some limitation is placed on the reduction of the bird's beak length.